Method for centrifugally removing liquid from a mixture

ABSTRACT

A method and apparatus for continuously removing, through a centrifuging operation, liquid from a mixture including therein a cellulose fiber pulp of organic origin which is used or could be used in the making of paper. The method involves impinging the mixture upon a rotating screen surface to form a layer of lightly compacted, relatively porous, fibrous material thereon and passing a portion of liquid through said porous layer of material and through said screen followed by effecting a further removal of the liquid from said mixture by cleaning said screen surface by moving the edge of a transportating member adjacent said screen surface while simultaneously producing and maintaining a mass of densely compacted, relatively impervious, fibrous material between said edge and said screen and using said mass to remove continuously from said screen surface fibrous material deposited thereon, and rolling, wringing and redepositing said fibrous material upon said screen surface while passing additional liquid through said screen and moving said fibrous material along said screen surface. The mixture is centrifuged at a force varying within a range between approximately 70 and 650 times the force of gravity. The fibrous material passing with the liquid through the screen is recovered by impinging the liquidfibrous material upon the surface of a second screen. The method may also be used with a mixture containing fibers which are relatively short in length by adding to said mixture a predetermined quantity of fibers having a relatively long length. The apparatus comprises a hollow, rotatably mounted screen means, a scroll member rotatably mounted concentrically within said screen means, and means for rotating the scroll member at a predetermined angular velocity differential relative to the angular velocity of rotation of the screen means. The screen means preferably has a circularly shaped inner surface and a plurality of openings formed therethrough, the minimum dimension of said openings, taken in a direction generally normal to the longitudinal axis of said openings, varying within a range of not less than approximately 0.3 millimeters nor more than approximately 3 millimeters. The scroll member has a plurality of helical flights formed on the surface thereof so constructed and arranged to prevent fibrous material from &#39;&#39;&#39;&#39;bridging&#39;&#39;&#39;&#39; between adjacent flights. This construction and arrangement includes helical flights disposed to form an included angle with a plane disposed perpendicular to the longitudinal axis of the scroll member not more than approximately 65*, and perpendicularly spacing each flight from an adjacent flight by an amount equalling at least approximately 0.03 and preferably 0.04 times the inner circumference of the screen means. The clearance between the edge of each flight and the inner surface of the screen means adjacent the end into which the mixture is supplied to the apparatus equals approximately not more than 0.004 times the inner diameter of the screen means. A relationship exists between certain structural parts of the apparatus whereby, upon operAtion thereof, the fibrous material is retained upon the inner surface of the screen means at least approximately 1 1/2 seconds and preferably about 3 seconds.

United States Patent [1 1 Wilson et al.

[ 1 June 4, 1974 1 1 METHOD FOR CENTRIFUGALLY REMOVING LIQUID FROM A MIXTURE [75] Inventors: Peter C. Wilson, Evergreen; Robert P. IIughart, Englewood, both of C010.

221 Filed: Aug.20, 1971 21 Appl.No.: 173,657

Related US. Application Data [621 Division of Ser. No. 752,022, Aug. 12, 1968, Pat. No.

[52] C1 210/78, 210/79, 210/213, 210/374, 210/380 [51] Int. Cl B04b 3/04 [58] Field Of Search 210/77, 78, 79, 213, 374, 210/380, 68

[56] References Cited UNITED STATES PATENTS 1,634,868 7/1927 Elmore 210/374 X 1,829,547 10/1931 Sharples.. 210/68 2,370,353 2/1945 Howe 1111 210/374 2,752,043 6/1956 Van Riel.... 210/78 2,870,912 1/1959 Mathieu 210/78 3,199,681 8/1965 Kirkpatrick... 210/374 3,348,767 10/1967 Ferney 210/374 X 3,642,139 2/1972 Wilson et a1 210/213 FORElGN PATENTS OR APPLICATIONS 1,216,088 12/1970 Great Britain Primary Examiner--Samih NJ Zaharna Assistant Examiner-F. F. Calvetti Attorney, Agent, or Firm-Duane Burton 57 ABSTRACT A method and apparatus for continuously removing, through a centrifuging operation, liquid from a mixture including therein a cellulose fiber pulp of organic origin which is used or could be used in the making of paper. The method involves impinging the mixture upon a rotating screen surface to form a layer of lightly compacted, relatively porous, fibrous material thereon and passing a portion of liquid through said porous layer of material and through said screen followed by effecting a further removal of the liquid from said mixture by cleaning said screen surface by moving the edge of a transportating member adjacent said screen surface while simultaneously producingand maintaining a mass of densely compacted, relatively impervious, fibrous material between said edge and said screenand using said mass to remove continuously from said screen surface fibrous material deposited thereon, and rolling, wringing and redepositing said fibrous material upon said screen surface while passing additional liquid through said screen and moving said fibrous material along said screen surface.

The mixture is centrifuged at a force varying within a range between approximately 70 and 650 times the force of gravity. The fibrous material passing with the liquid through the screen is recovered by impinging the liquid-fibrous material upon the surface of a second screen. The method may also be used with a mixture containing fibers which are relatively short in length by adding to said mixture a predetermined quantity of fibers having a relatively longlength. The apparatus comprises a hollow, rotatably mounted screen means, a scroll member rotatably mounted concentrically within said screen means, and means for rotating the scroll member at a predetermined angular velocity differential relative to the angular velocity of rotation of the screen means. The screen means preferably has a circularly shaped inner surface and a plurality of openings formed therethrough, the minimum dimension of said openings, taken in a direction generally normal to the longitudinal axis of said openings, varying within a range of not less than approximately 0.3 millimeters nor more than approximately 3 millimeters. The scroll member has a plurality of hellcal flights formed on the surface thereof so constructed and arranged to prevent fibrous material from bridging between adjacent flights. This construction and arrangement includes helical flights disposed to form an included angle with a plane disposed perpendicular to the longitudinal axis'of the scroll member not more than approximately 65, and perpendicularly spacing each flight from an adjacent flight by an amount equalling at least approximately 0.03 and preferably 0.04 times the inner circumference of the screen means. The clearance between the edge of each flight and the inner surface of the screen means adjacent the end into which the mixture is supplied to the apparatus equals approximately not more than 0.004 times the inner diameter of the screen means. A relationship exists between certain structural parts of the apparatus whereby, upon operation thereof, the fibrous material is retained upon the inner surface of the screen means at least approximately 1 /2 seconds and preferably about 3 seconds.

13 Claims, 17 Drawing Figures PATENTEDJUN 41974 SHEUZOFT FATENTEDJUH 4 m4 sum u or 7 PATENTEDJuR 4-1914 SHEET 5 (1r 7 FEED VOLUME- GPM STOCK TPATENTEDJUN 41914 SHEEI 7 OF 7 o 5 O 5 O 5 O 5 O 5 O H Y RT AN DE NU 0L CF EF SE METHOD FOR CENTRIFUGALLY REMOVING LIQUID FROM A MIXTURE This is a division of application Ser. No. 752,022, filed on Aug. 12, 1968 now U.S. Pat. No. 3,642,139.

BACKGROUND OF INVENTION from a dilute fonn, as-low asone-half of 1 percent to 5 percent consistency (percent by weight of fibrous solids contained in the mixture) to a thickened form as high as 6 percent to 40 percent consistency, anywhere from four to ten times before ending up as a product from which paper can be made. Since cellulose fiber pulps originate from different organic species and from different chemical or mechanical processes, they exhibit varying physical characteristics typical of which is the ability to dewater. For example, in a given type of dewatering equipment, different pulps will dewater to a range of consistency, hence the use of ranges of per cent solids herein. At the present time in the United States approximately 40 million tons of cellulose fiber pulp of organic origin, and which is useful in the papermaking industry, are produced each year. However, due to the fact that a pulp may be processed or cycled a number of times before ending up as a finished product, liquid is extracted from as much as 3 to billion tons of dilute pulp mixture in the United States alone each year. It will therefore be apparent that the method of such removal is a formidable problem deserving of technical attention and efforts to accomplish dewatering, thickening or washing in a more economical or expeditious manner. Further, as in any washing system employing a counter-current action involving several repeated steps of dewatering, repulping and subsequent dewatering, there are numerous advantages inherent in dewatering to the highest possible consistency. Although it is generally recognized that where applicable a centrifuge is an efficient mechanical device to use for removing liquid from a mixture, with the exception of the invention herein described and with the exception of solid bowl types of centrifuges which are used to thicken to low consistency paper and pulp mill wastes consisting of relatively fiber free mixtures containing fillers such as clay, starch, titanium oxide and other extraneous particulate (not fibrous) types of solids, not a single continuous type of centrifuge is known to be used in the United States or elsewhere to remove liquid from any dilute form of fibrous pulp material useful in the making of paper or similar cellulose fiber products.

The types of appurtenances being used to remove liquid from fibrous pulp material in dilute form are presses, gravity and vacuum filters, stationary, vibrating, rotating, or traveling screens, and hybrids of any combination of these. Presses are categorized into types including screw presses, disc presses, belt presses and' roll presses. The production capacity of these presses is greatly enhanced through the use of an apparatus which increases the consistency of the material before same is fed to any of the presses and, thus,

presses are frequently used in combination with such an apparatus. Screw presses are disadvantageous since the fibers often become permanently kinked and entangled with each other producing dense fiber bundles. The existence of dense fiber bundles in pulp makes such pulp relatively undesirable for the manufacture of quality paper thereby decreasing the value of pulp dewatered in this manner. Although the consistency of pulp produced by presses is relatively high, approximately 30 percent or more, it is generally recognized that presses are disadvantageous from the acquisition cost thereof as compared to their production capacity and, also, frequently from a maintenance viewpoint.

There are two general types of filters namely, gravity filters and vacuum filters. The gravity filter comprises a cylindrically shaped screen rotatably mounted within a vat. The pulp mixture is fed into the vat. The major portion of the liquid within the mixture passes through the screen surface into the center of the circularly shaped screen where same is withdrawn. The

fibrous material is retained on the outer surface thereof and is removed through the use of scrapers or the like. The vacuum filter is similar to a gravity filter but has the interior of these cylindrical screens exposed to a vacuum source for the purpose of increasing the rate of liquid flow through the screen surface. Because of the high air-liquid flow through the vacuum filter, the vacuum source is for economic reasons usually obtained by means of a barometric leg. To obtain the barometric leg the vacuum filter must be installed in a position considerably elevated above normal ground level. The gravity filter is capable of handling pulp of approximately one-half of 1 percent to 1% percent consistency to produce a pulp of approximately 6 percent to 7 percent consistency. A vacuum filter, with suitable accessories such as pressure rolls and the like, is capable of handling pulp of approximately one-half of 1 percent to .l% percent consistency to produce a pulp of 10 percent to 15 percent consistency.

An example of a hybrid device used to thicken pulp, and which is a combination of filter and roll press, comprises four vertically disposed drums located about a central point and each of which is disposed in rotating contact with two other drums. The walls of the drums are perforated and-covered by screen plates. The four drums comprise two pairs of drums, each drum of each I pair cooperating with the other drum of said pair. The pulp mixture is fed into the pressurized area circumscribed by the four drums and some of the liquid passes through the screen surfaces into the center of each drum depositing a layer of fiber on the surface of each drum. The fiber then passes between the surfaces of both of the pairs of cooperating'drums. Due to the squeezing action exerted upon the fiber as same passes between said drums, an additional portion of the liquid is removed from the fiber. Although this hybrid device may be used to process pulp of consistencies lower than 9 percent to 12 percent, such use is normally avoided Centrifugal Screening devices are used in the paper making industry, to remove shives and other foreign particles from liberated fibers. Such devices are not used to thicken the fibrous pulp but instead are designed to remove foreign material frompulps having a consistency of approximately 2 percent or less.

Although considerable effort has been expended in the past to produce a centrifugal apparatus capable of removing liquid from a cellulose fiber pulp of organic origin which isused in or useful in making paper, all such efforts have been. relatively unsuccessful. Generally, prior art methods and apparatuses involving the centrifuging of a cellulose pulp as aforedescribed, have been unacceptablefor commercial use due to the extremely; small amounts of pulp that can be processed therethrough. There are a number of reasons why prior art methods and apparatuses involving centrifuging of pulp have not been successful. One of the problems encountered involved the utilization of a satisfactory screen surface. If the diameter of the screen openings were made too large, there would be too great a loss of fiber through the screen and, additionally, the ten dencyfor the screen to blind increases with an increase in the size of screen openings. Thus, attempts were made to centrifuge pulp while using thin screen surfaces having very small openings. However, screens having small sized openings and a nominal percentage of open area were too fragile; thus, the use of such a screen member was disadvantageous since it had an extremely short life of sometimes only a few hours.

Another problem encountered with centrifuges was that of preventing the fibrous material from bridging between adjacent flights whereby fibrous material no longer passed through the apparatus. The basket angles of prior art centrifugal devices used in attempts to dewater fibrous pulps of organic origin utilized in the paper making industry were such that excessive gravity forces were produced in a direction downward of the screening surface with a resultant loss of control of dilute pulp within the rotating basket.

Gravity forces developed by prior art centrifugal dewatering devices utilized 'in attempts to thicken the paper pulps,were of sufficient magnitude so as to frequently form a thin, relatively impervious layer of fiber so close to the screen surface that the co-rotating scrapers were unable to reach and remove this dense fiber layer from the rotating screen surface. Additionally, pulp dewatering was greatly restricted due to blockage of the screen surface by the dense layer of fii totally unsatisfactory.

SUMMARY OF THE INVENTION It has been found that each of the foregoing disadvantages of the prior artmethods and apparatuses for removing liquid from a cellulose pulp of organic origin which is useful in the paper making industry through centrifuging of same can be overcome through the use of the method and apparatus of the subject invention.

4 The method of this inventioncomprises the steps of impinging a cellulose pulp of organic origin which is useful in the paper making industry upon a rotating screen surface to form a layer of lightly compacted, relatively porous fibrous material thereon and passing a portion of the liquid through the porous layer of material and the screen, and effecting a further removal of the liquid from said pulp by cleaning said screen surface by moving the edge of a transporting member adjacent the screen surface while simultaneously producing and maintaining a mass of densely compacted, relatively impervious, fibrous material adjacent one side of said edge of said transporting member and between said surfaces and using said mass to remove continuously said layer of fibrous material from said screen surface, and rolling, wringing and redepositing said fibrous material upon said screen surface while passing additional liquid through said screen and moving said fibrous .material along said screen surface. The fibrous material is subjected to said centrifugal force for a predetermined period of time of at least approximately one and onehalf seconds or longer. The centrifugal apparatus of this invention comprises a hollow, rotatably mounted screen means of .predetermined length having an inner surface which, in cross-section, forms with a line disposed normal to the longitudinal axis of said screen means an included angle varying between approximately and 105 and is preferably formed at an angle of approximately said screen means having a plurality of openings formed therethrough, the minimum dimension of said openings, tak'en in adirection generally normal to the longitudinal axis thereof, varying within a range of not less than approximately 0.3 millimeters to not more than approximately 3 millime ters; a rotatably mounted scroll member concentrically mounted within said screen means, said scroll member having a plurality of helical flights formed on the surface thereof, at a predetermined angle of inclination each flight having an edge and a fibrous material engaging surface, the distance separating said edge of each flight and the inner surface of said screen means adjacent at least a first end of said screen means equalling approximately not more than 0.004 times the inner diameter of the screen means, the surface of said flight being smoother than the inner surface of said screen means, the' flights being so constructed and arranged to prevent bridging of fibrous'material between adjacent flights; means for rotating said scroll member at a predetermined angular velocity differential relative to the angular velocity of rotation of said screen means; the relationship between the length of the screen means, the amount of angular velocity, the angle of inclination of said helical flights and the smoothness of the flight surfaces being maintained such that, during operation of the apparatus, the fibrous material is retained upon the inner surface of the screen means at least approximately one and one-half seconds; feed inlet means; liquid receiving means; and fibrous material receiving means.

Accordingly, one of the principal objects of this invention is to provide an improved method and apparatus for continuously removing liquid from a cellulose pulp of organic origin which is useful in the paper making industry by centrifuging same.

Another object of this invention it to provide a method of continuously removing liquid from a low n consistency organic cellulose pulp mixture by centrifuging same.

Another object of this invention is to provide a method of continuously removing liquid from a cellulose fiber mixture of organic origin comprising the step i of subjecting the cellulose fiber mixture to a centrifugal force varying within a range between approximately 70 and 650 times the force of gravity by impinging said mixture against a rotating screen surface until a portion of the liquid contained within said mixture passes through said screen, and effecting a further removal of liquid from said mixture and passing same through said screen by continuously removing said fiber from said.

screen surface, rolling and wringing said fiber, and redepositing same upon said screen surface while moving said fiber along said screen surface.

Another object of this invention is to provide an improved method as aforedescribed in which the mixture is centrifuged at an accelerativeforce varying within a range of approximately 100 to 550 times the force of gravity. Another object of this invention is to provide an improved method as aforedescribed in which the mixture is centrifuged. at an accelerative force varying within a range between approximately 140 and 450 times the force of gravity. I

Another object of this invention is to provide an improved method as aforedescribed including the step of recovering substantially all of the fiber passing with the liquid through said screen by impinging said liquidfibrous. material mixture upon the surface of a second screen.

Another object of this invention is to provide an improved method as aforedescribed in which the step of impinging said liquid-fibrous material upon the surface of said second screen includes moving said material toward the surface of said second screen at an angle obliquely disposed with respect to the surface of said second screen.

Another object of this invention is to provide an improved method as aforedescribed including the step of treating or washing said fibrous material with a liquid or treating said fibrous material with a gas following the removal of substantially all of the liquid that is to be removed therefrom and moving substantially all of said gas along with said fibrous material.

Another object of this invention is to provide a method of centrifugally removing liquid from a mixture including therein a fibrous material the fibers of which are relatively short in length, as characterizedas being produced by a two or three pass stone or refiner.

groundwood process, said method comprising the steps of adding to said mixture a predetermined quantity of fibers which are relatively long in length, impinging said mixture upon a rotating screen until substantially all of the fibrous material contained therein forms a layer on said surface while passing a portion of said liquid through said screen, and effecting a further removal of the liquid from said mixture by cleaning said screen by moving the surface of a transporting member adjacent said screen surface while simultaneously producing and maintaining a mass of densely compacted, relatively impervious fibrous material adjacent one side of said member and between said surfaces and passing additional portions of liquid through said screen, the

quantity of fibers having a relatively long length being sufficient to effect the producing and maintaining of said mass of densely compacted, relatively impervious fibrous material.

Another object of this invention is to provide a centrifugal apparatus forv removing liquid from a mixture,

said apparatus including a hollow, rotatably mounted screen means and a scroll memberrotatably mounted concentrically within said screen means,-said screen means including an inner screen member and an outer encompassing perforated basket, said scroll member having a plurality of helical flights formed on the surface thereof, the improvement comprising a generally longitudinally extending recess formed on the inner surface of said basket, said screen member being formed from a layer of material and having opposite ends thereof secured together in overlapping relation, said overlapping ends being disposed within said recess whereby the inner surface of said screen member is substantially smooth throughout the peripheral extent thereof.

Another object of this invention is to provide a centrifugal apparatus for removing liquid from a mixture having a fibrous material therein, said apparatus including a hollow, rotatably mounted screen means and a scroll member rotatably mounted concentrically within said screen means, said screen means including an inner screen member and an outer encompassing perforated basket, said scroll member having a plurality of helical flights formed on the surface thereof, the improvement comprising a second screen means mounted generally concentrically of and in spaced apart relation to said screen means and exteriorly thereof.

Another object of this invention it to provide a centrifugal apparatus as aforedescribed in which said second screen means is stationarily mounted.

Another object of this invention is to provide a centrifiigal apparatus as aforedescribed in which the inner surface of the second screen means is conically shaped.

Another object of this invention is to provide a centrifugal apparatus for continuously removing liquid from a cellulose pulp of organic origin which is useful in the paper making industry, said apparatus comprising a hollow, rotatably mounted screen means of predetermined length, said screen means having an axially extending inner surface preferably substantially constant in radius and having a first and secondend, said screen means having a plurality of openings formed therethrough, the minimum dimension of said openings, taken in a direction generally normal to the longitudinal axis thereof, varying within a range of not less than approximately 0.3 millimeters to not more than approximately 3 millimeters, a rotatably mounted scroll member concentrically mounted within said screen means, said scroll member having a first and second end, said first and second ends being disposed, respectively, adjacent said first and second ends of said screen means, said scroll member having a plurality of helical flights formed thereon at a predetermined angle of inclination, each flight having an edge and a fibrous material engaging surface, the perpendicular distance separating each edge of flight and the inner surface of said screen means adjacent at least said first end thereof equalling approximately not more than 0.004 times the inner diameter of said screen means, the surface of said flight being smoother than the inner surface of said screen means, the helical flights being so consaid scroll member at a predetermined angular velocity differential relative to the angular velocity of the rotation of said screen means, the relationship between the length of said screen means, the amount of angular velocity differential, the angle of inclination of said helical flights and the smoothness of the flight surfaces relative to the inner surface of the screen means being maintained such that during operation of the apparatus the fibrous material is retained upon the inner surface of said screen means at least approximately 1% seconds, feed inlet means, liquid receiving means, and fibrous material receiving means.

Another object of this invention is to provide an apparatus as aforedescribed in which the size of openings formed in the inner surface of said screen means varies within a range of approximately 0.5 millimeters to approximately 2 millimeters, and preferablyv Within a range of approximately I millimeter to approximately 1.68 millimeters.

Another object of this invention is to provide a centrifugal apparatus as aforedescribed in which the size of openings formed in the inner surface of said screen means varies within a range of approximately to 50 times the diameter of the fibers contained within the mixture.

Another object of this invention is to provide an apparatus as aforedescribed in which said scroll member has more helical flights formed adjacent the first end thereof than adjacent the second end thereof.

Another object of this invention is to provide an apparatus as aforedescribed inwhich the edges of said flight are composed of an elastomeric material such as polyurethane, polyethylene, nylon, rubber, silicone rubber, and polyvinyl chloride.

Another object of this invention is to provide a centrifugal apparatus as aforedescribed including a second screen means mounted generally concentrically of the rotatably mounted screen means.

Another object of thisinvention is to provide a centrifugal apparatus'as aforedescribed in which the minimum dimension of the openings of said second screen means, taken in a direction generally normal to the longitudinal axis thereof, is less than the corresponding minimum dimension of the openings of said rotatably mounted screen means.

Another object of this invention is to provide a centrifugal apparatus as aforesaid in which the construction and arrangement to prevent bridgingof fibrous material between adjacent flights includes perpendicularly spacing each flight from an adjacent flight adjacent at least the second end of said scroll member by an amount equalling at least approximately 0.03 and preferably 0.04 times the inner circumference of said screen means and said flights form an included angle with a plane disposed perpendicular to the longitudinal axis of said scroll member of not more than approximately 55 and preferably not more than approximately 45.

BRIEF DESCRIPTION OF THE DRAWINGS and claims and illustrated in the accompanying drawings, wherein:

8 FIG. 1 is an elevational view, partially cut away and in partial cross-section, showing an apparatus constructed in accordance with the subject invention;

FIG. 2 is an enlarged cross-sectional view of the screen means shown in FIG. 1;

FIG. 3 is a schematic view, in cross-section, taken perpendicular to the longitudinal axis of the screen means adjacent the end receiving material to be processed therethrough and showing a layer of lightly compacted, relatively porous, fibrous material deposited upon the inner surface of said screen means and the mass of densely compacted, relatively impervious fibrous material formed adjacent one surface of the heli cal flight of said scroll member and between the edge of said flights andthe inner surface of said screen means; i

FIG. 4 is a view taken similar'to that of FIG. 3 showing the recess formed on the inner surface of the basket of said screen means and the overlapping ends of the inner screen member disposed within said recess;

FIG. 5 is a schematic view showing the angle of impingement upon the inner surface of a second screen means of the material passing through the inner or first screen means;

FIGS. 6-9 are cross-sectional views of a helical flight and a portion of the scroll member to which same is attached;

FIGS. 10 and 11 are views in partial cross-section of conically shaped scroll members and screen means;

FIG. 12 is a view, in partial c ross-section, of a scroll member and screen means wherein the first end of the screen means has-a conically shaped inner surface the diameter of which decreases in a direction proceeding toward the second end, and the second end has a conically shaped surface the diameter of which decreases in a direction proceeding toward said first end;

FIG. 13 is a view similar to that shown in FIG. 12 except' that the first end has an axially extending inner surface of substantially constant radius;

FIG. 14 is a graph showing the relationship between the quantity of pulp processed through an apparatus constructed in accordance withthe subject invention to the consistency of the discharged product;

FIG. 15 is another graph showing the optimum accelerative forces at which the pulp is to be centrifuged and related power requirements for maximum product dryness at a given rate of feed;

FIG. 16 is another graph showing clarification of the centrifuged effluent obtained by use of a second screen means; and

FIG. 17 is another graph showing the results obtained by adding long fibered material to low consistency.

short fibered pulp in dilute form.

DESCRIPTION OF THE PREFERRED.

- EMBODIMENTS dance with the subject invention. The centrifugal apparatus 20 comprises a hollow, rotatably mounted screen means 22 of predetermined length, a second screen means 23 preferably stationarily mounted, a rotatably mounted scroll member 24, feed inlet means 26, means 28 located exteriorly of said screen means 22 for receiving the liquid to be removed from the mixture, and fibrous material receiving means 30. The centrifugal apparatus 20 has an upper casing member 32 and a lower casing member 34. The scroll member 24 is cylindrically shaped and is concentrically mounted upon a shaft 36 for rotation therewith. The screen means 22 is securely attached to a hollow shaft 40. The shaft 36 is concentrically mounted within the hollow shaft 40 for relative rotation with respect to hollow shaft 40. The shaft 40 passes through a pair of spaced apart bearings (not shown). The shaft 40 is driven by V-belts 41 and has a gear 42 securely attached thereto for rotation therewith. The gear 42 attached to shaft 40 drives'a mating gear 42A which is mounted on a shaft 43. A second gear 44 is mounted on shaft 43 for rotation therewith and is disposed in driving engagement with a gear 45 securely attached to the shaft 36. By suitable selection of the gears, one of the shafts 40 may be operated at a predetermined angular velocity differential relative to the angular velocity of the other shaft 36. The gear 42 that is securely attached to the shaft 40 is driven by the motor 46. The gears referred to above are mounted within the gear box 47. The lower casing member 34 has an opening 48 which, if desired, may be connected to a vacuum source (not shown).

The hollow, rotatably mounted screen means 22 preferably comprises an inner screen member 49, an outer encompassing perforated basket 50 and a layer of woven backup wire 51 sandwiched therebetween, see FIG. 2. The inner screen member 49 has a plurality of openings 52 formed therethrough. The minimum dimension of said openings, taken in a direction generally normal to the longitudinal axis of the opening, varies within a range of not less than approximately 0.3 millimeters nor more than approximately 3 millimeters. Good results have been obtained where the minimum dimension of said openings varies within a range of not less than approximately 0.5 millimeters nor more than approximately 2 millimeters and excellent results have been obtained where the minimum dimension varies within a range of not less than approximately 1 millimeter nor more than approximately 1.68 millimeters. Understandably, the size of opening is related to the diameter of the fiber within the mixture being processed through the apparatus. Consequently, it is preferred that the minimum dimension of said openings vary within a range of not less than approximately l nor more than approximately 50 times the diameter of the fibers contained within the mixture. The effective open area of the inner screen member 49 should be as large as possible and should be at least percent or greater. Additionally, in order to obtain a long operating life it is desired that the inner screen member 49 be able to resist the abrasive action of the fibrous material moving along the inner surface of the screen means. Such abrasive action becomes pronounced following removal of sufficient liquid to increase the consistency to approximately 8 percent or more. Consequently, long life for a screen member 49 is obtained by making same as thick as possible. It-has been found that these objectives are attained by utilizing an inner screen member having openings 52 formed as aforedescribed and wherein the thickness of said inner screen member49 approximately equals said minimum dimension of said openings. It will be understood that the backup woven wire 51 provides support to the inner screen member 49 and the perforated basket 50 provides good support to both the backup wire 51 and the inner screen member 49. a

As previously indicated, the rotatably mounted scroll member 24 is generally cylindrical in shape. The scroll member 24 is axially mounted within the screen means 22. The scroll member 24 has first and second ends 54 and 56. These ends 54 and 56 are disposed, respectively, adjacent first and second ends 58 and 60 of the screen means 22. The scroll member 24 has a plurality of helical flights 62 formed thereon at a predetermined angle of inclination and extending from one end to the other. The scroll member 24 may also have additional flights 64 formed on the surface thereof adjacent the first end 54. The flights on the scroll member 24 are constructed and arranged to prevent bridging of fibrous material between adjacent flights, said construction and arrangement comprising perpendicularly spacing,

, adjacent the second end of said scroll member or that portion of the scroll member in contact with a fibrous material mixture no longer in liquid form such as normally is the case where the consistency equals approximately 8 percent or more, each flight from an adjacent flight by an amount equalling at least approximately 0.03 and preferably 0.04 times the inner circumference of said screen means and said flights forming an included angle with a plane disposed perpendicular to the longitudinal axis of the scroll member of not more than approximately 55 (for a cylindrically shaped scroll member and screen means) and preferably not more than approximately 45. At the present time, it is preferred that said included angle vary within a range of approximately 10 to 15.

For certain applications it has been found desirable to form the flights or at least coat the edges thereof with a material which has a relatively low coefficient of friction and is relatively rigid but ablative in character. As shown in FIG. 6, the helical flight 66 may be formed from a suitable elastomeric material, such as polyurethane, polyethylene, nylon, rubber, silicone rubber and polyvinyl chloride, and secured to the scroll member 24 in any suitable manner such as with threaded bolts 68 (only one of which is shown). In FIG. 7 is shown a helical flight 70 comprising a member 72 integrally formed to the scroll 24 and a layer of material 74 securely attached to a substantial surface portion of the member 72. The edge 76 of the helical flight 70 is arcuately formed as contrasted to the planar edge 78 shown in FIG. 6. in FIGS. 8 and 9 are shown helical flights 80 and 82 each of which comprise a member 72 integrally formed with the scroll 24 and a layer of elastomeric material. The layer 84 of elastomeric material shown in FIG. 8 is attached to one surface of the member 72 and projects beyond the radial end thereof by a predetermined, relatively short distance. The layer 86 of the material shown in FIG. 9 is, in cross-section, generally U-shaped in configuration. The use of a layer of material as aforedescribed greatly facilitates the forming of a helical flight which, during rotation, circumscribes a configuration substantially similar to the inner surface of said screen means. For example, it is necessary that the clearance between the edges of the helical flights and the inner screen surface be maintained within close predetermined limits. Where conical configurations are involved this is effected merely by moving the screen means axially and, thus, toward or away from the scroll member. However, this technique cannot be used for cylindrical configurations. In order to overcome this problem and to reduce manufacturing costs, the edges of the helical flights are coated with an 1 l elastomeric material as aforesaid until the overall diameter thereof approximately equals the inner diameter of the screen means. The scroll member is then assembled within the centrifugal apparatus and rotated until the elastomeric material has been worn away sufficiently apparatus. Additionally, in the event the edges of the helical flight experience excessive wear, it is a relatively simple matter to remedy same by removing the remaining portion of the layer of material and replacing same with a new layer of material.

Each flight includes a fibrous material engaging surface such as surface 67 of helical flight 66 shown in FIG. 6 or surface 75 of helical flight 70 shown in FIG. 7. The fibrous material engaging surfaces are smoother than the inner surface of the screen means. More specifically, the coefficient of friction of the fibrous material to the fibrous material engaging surface of the flight is less than the coefficient of friction existing between the fibrous material and the inner surface of the screen means. Due to the fact thata plurality of openings are formed on the inner surface of the screen means, it will be understood that the coefficient of friction existing between fibrous material and the inner surface of the screen means will normally be higherthan the coefficient of friction existing between fibrous material and the fibrous material engaging surface which is formed relatively smooth. A predetermined relationship must be maintained between the length of the screen means, the amount of angular velocity differential between the screen means and the scroll member, the angle of inclination of the helical flights and the smoothness of the fibrous material engaging surfaces of the flights relative to the inner surface of the screen means such that, during operation of the apparatus, the fibrous material is retained upon the inner surface of the screen means at least approximately l k seconds. The ability of an apparatus constructed in accordance with the subject invention to remove liquid from a fibrous material in an efficient manner will beadversely effected where the retention time is materially less than that as afores'pecified.

in FIG. 4 is shown a modified screen means 88 comprising an inner screen member 90 and an outer encompassing perforated basket 92. The basket 92 hasa generally longitudinally extending recess 94 formed on the inner surface thereof. The inner screen member 90 means located exteriorly of screen means 22. The reason for using a second screen means is to recover a substantial portion of the fibrous material which passes with the liquid through the screen means 22. In FIG.

12 5 is schematically shown a second screen means 100 which is mounted generally concentrically to the screen means 22 and exteriorly-thereof. ln order-to maximize the amount of fiber being recovered on the inner surface of the second screen means 100 it is required that the material passing through the screen means 22 impinge upon the inner surface of the second screen means 100 in a direction non-perpendicularly disposed with respect to the inner surface of said second screen means and preferably at an oblique angle with respect to the surface of said second screen means such as an angle varying between approximately 20 to 30. Since the screen means 22 is rotated at a relatively rapid angular velocity in the direction indicated by the arrow 102 it is possible to effect the foregoing merely by mounting the second screen means 100 stationarily within the apparatus 20. The arrows 104 indicate the trajectory or path of travel of the material following exiting of same from the screen means 22.

Although it is preferred that the scroll memberscreen means assembly used in an apparatus of this invention preferably be cylindrical in configuration, the results of previous work show that satisfactory results can be obtained where the screen means is formed with an inner surface which, in cross-section, forms with a line disposed normal to the longitudinal axis of the screen means an included angle varying between approximately and 105. The included angle of the screen means 106 shown in FIG. 10 is approximately 83. The mixture to be processed through the machine is fed between the screen means 106 and the scroll member 108 at the upper end thereof in the direction indicated by the arrow 110. The fibrous material exits .from said assembly in the direction indicated by arrows 112. In FIG. 11 is shown an assembled screen means 114 and scroll member 116. The inner surface of the screen means 114 forms with a line disposed normal to the longitudinal axis of said screen means an included angle of approximately 97. In FIG. 11, the material to be processed through the apparatus is fed between the screen means and the scroll member in the direction indicated by the arrow 118' while the fibrous material exits from said assembly in the direction indicated by the arrows 119. .With respect to the assemblies shown in FIGS. 10 and 11, it will be understood that in both instances the screen means must rotate at a slightly faster angular velocity in a counter clockwise direction (or a slightly slower angular velocity when rotated in a clockwise direction), taken with respect to a plan view thereof, than the scroll member in order for the material to be processed through said assembly in the manner as aforedescribed. Although the longitudinal axis of the assembly shown in FIGS. 10 and 11 is disposed in the preferred vertical position, it is to be understood that beneficial results may be obtained when the longitudinal axis of an apparatus constructed in accordance with the subject invention is disposed vertically or horizontally or in any other position.

The results of work which has been done to date show that the screen means and scroll member may be formed with a hybrid configuration such as that shown in FIGS. 12 and 13. For example, FIG. 12 shows a screen and scroll member assembly wherein the first end of the screen means has an inner surface which, in cross section, forms with a line disposed normal to the longitudinal axis of the screen means an included angle varying between approximately and while the second end has an inner surface which, in crosssection, forms with a line disposed normal to the longi tudinal axis of the screen means an included angle varying between 75 and 90. As shown, the down stream portion of the first end. taken in the direction of flow of fibrous material mixture therethrough. is contiguous with the upstream portion of the second end. In FIG. 13, is shown a screen means and scroll member assembly wherein the first end of the screen means has an axially extending inner surface of substantially constant radius while the second end has an inner surface which, in cross'section, forms with a line disposed normal to the longitudinal axis of the screen means an included angle varying between approximately 75 and 90. It is preferred that the longitudinal length of the first end of the screen means'shown in either FIGS. 12 or 13 should be of a sufficient length to produce a fibrous material thereon having a consistency of at least approximately 8 percent. The screen means and scroll member assemblies shown in FIGS. 12 and 13 are mounted, respectively, within second screen means 120 and 121.

FIG. 3 is a schematic view, in cross-section, taken perpendicular to the longitudinal axis of the screen means 122 adjacent the end thereof receiving material to be processed therethrough. In FIG. 3 is shown a layer 124 of lightly compacted, relatively porous fibrous material deposited upon the inner surface of said screen means and extending a substantial distance between adjacent flights 126 which are integrally formed on the scroll member 128. Additionally, in FIG. 3 is shown a mass 130 of densely compacted, relatively impervious fibrous material formed adjacent the leading surface or fibrous material engaging surface 132 of each helical flight 126 and between the edge 134 of each flight and a corresponding portion of the inner surface of said screen means 122. A portion of the mixture being processed through the apparatus is represented generally by the numeral 136. The arrows 138 represent, quantitatively, the relative amount of liquid exiting from the outer surface of the screen means 122. For all practical purposes, no liquid exits through the screen means 122 which is in contact with the mass 130 of densely compacted, relatively impervious fibrous material. As viewed in FIG. 3, the amount of liquid passing through the layer 124 of lightly compacted, relatively porous fibrous material decreases as the thickness of said layer increases and is a maximum where the fibrousmaterial has been cleaned from the screen surface by one of the masses 130. It is important that the degree of compaction of the layer 124 upon the inner surface of.the screen means 122 be controlled suffi ciently to permit liquid to pass therethrough if sufficient removal of liquid, particularly for high feed rates, is to be obtained. It will be understood that since the screen means 122 is moving in a counterclockwise direction slightly faster than the angular velocity of the scroll member 128, the leading surfaces 1320f the flights 126 will contact the fibrous material deposited upon the inner surface of the screen means 122 as shown in FIG. 3. Thus, each flight 126 upon initially 6 coming into contact with the fibrous material deposited upon the inner surface of the screen means 122 further compacts the fibrous material at the point of contact or impact, removes same from the inner surface of the screen means, and operates to effect a rolling and wringing action thereof which further decreases the amount of liquid contained therein before the fibrous material is redeposited upon the inner surface of the screen means following which the foregoing action is repeated until the fibrous material exits from between the scroll member and the screen means. The portion 124A of layer 124 represents that fibrous material from which additional liquid has been removed as a result of the rolling and wringing action as aforedescribed. (onsequently, it will be appreciated that as the fibrous inaterial moves along the inner surface of the screen means, liquid will continue to be removed from the fibrous material although the amount being removed will decrease as the action continues.

It is important that control of the fibrous material deposited upon the inner surface of the screen means be maintained at all times if optimum results are to be realized. For example, with prior art methods and apparatuses in which centrifuging was involved, it was found that under certain conditions the fibrous material would become tightly wedged between adjacent flights and could no longer be moved longitudinally of the screen means. This problem normally occurred after a predetermined amount of liquid had been removed from the fibrous material, normally when the consistency of the material exceeded approximately 8 percent. The occurrence of this bridging condition would stop up the apparatus thereby necessitating a complete shut down in order to remove the fibrous material from between the flights. It has been found, however, that means may be incorporated within an apparatus of this invention to preclude this condition. For a cylindrically shaped screen means and scroll member such means includes forming the helical flights in a manner whereby same form an included angle with a plane disposed perpendicular to the longitudinal axis of the scroll member of not more than approximately and preferably not more than approximately 45. At the present time, the most desired angle to be used is one varying between approximately 10 and 15. The tendency of fibrous material to bridge between adjacent flights is further minimized by maintaining, at least adjacent the second end of the scroll member, the perpendicular distance between successive flights adjacent equal to at least approximately 0.03 and preferably.

0.04 times the inner circumference of the screen means.

The included angle formed between the helical flights and a plane disposed perpendicular to the longitudinal axis of the scroll member varies somewhat from the above criteria where conically shaped screen means and scroll members are involved. For example, where .a conically shaped screen means and scroll member similar to that shown in FIG. 10 are involved, the included angle may be increased up to a maximum of approximately as the configuration of the inner surface of the screen means changes from an axially ex tending surface of substantially constant radius to a conically shaped member. Where a conically shaped screen means and scroll member similar to that shown in FIG. 11 are involved, the maximum included angle is decreased from 55 down to approximately 45.

0 More specifically, as the configuration of the inner surface of the screen means changes from an axially extending surface of substantially constant radius to a conically shaped member, the aforesaid included angle will decrease proportionally. In connection with the foregoing, it will be understood that effecting and maintaining control of the fibrous material deposited upon the inner surface of the screen means increases in difficulty when the configuration of the inner surface of the screen means departs from an axially extending surface of substantially constant radius to a conical shape the angle of which varies between 75 and 90. However, the problem of effecting control is decreased where a configuration of the inner surface of the screen means changes from an axially extending surface of substantially constant radius to a conical shape the angle of which varies from 90 to 105. It will be understood that with the latter type of configuration more power is required to operate such an apparatus and. thus, the efficiency thereof is less. In view of the foregoing, it will be appreciated that the screen means and scroll assembly shown in F 1G. 13 not only permits good control of the fibrous material along the screen means but also enables operation of same with reduced power consumption.

As previously indicated, the clearance maintained; between the edges of the helical flights and the opposed inner surface of the main screen means, at least adjacent the end into which the fibrous material mixture is suppliedto the main screen means and scroll member, equals approximately not more than 0.004 times the inner diameter of the screen means. Where the centrifugal force at which the fibrous material mixture is centrifuged is increased, it will be understood that the clearance factor becomes increasingly more' important and, further, the amount of clearance should'preferably be correspondingly decreased. Moreover, the importance of the amount of clearance involved decreases as the material proceeds toward the outlet end of the screen means. By this it is meant that the amount of clearance may increase from the inlet end toward the outlet end without unduly affecting the results obtained from an apparatus constructed in accordance with this invention. Itwill also be understood that the amount of clearance is also dependent, to some extent, upon the length of fibers contained within the fibrous material mixture. In this connection, the amount of clearance is somewhat proportional to the length of the fibers in volved, i.e., a larger clearance may be used for a longer fiber length.

FIG. 14 is a graph showing the relationship between quantity of pulp processed through an apparatus constructed in accordance with the subject invention to the consistency of the product discharged from said apparatus. The apparatus used in obtaining the data shown in FIG. 14 was constructed generally in accordance with the apparatus shown in FIG. .1 except that the relatively short flights 64 were omitted and the opening 48 either was omitted or was not connected to a source of vacuum. The screen means 22 was 48 inches high and had an inner diameter of 36 inches. The scroll member 24 and 16 flights each of which formed .an included angle with a plane disposed perpendicular to the longi-' tudinal axis of the scroll member of approximately40 The following is a tabulation of test data obtained with Unbleached SOItwood Kraft Bleached Softwood Sulfite f 3. 02 693 120 308 10 2s. 7 3. 02 693 120 30s 10 2s. a a 02 693 120 30s 10 37. 6 z 84 695 50 187 s 25. s 2 84 695 50 187 a 26. 2. 84 695 50 187 8 28. 7 a 84 695 50 is? s as 0 Bleached Softwood Kraft 700- a 66 677 .50 247 0 24. 295. 3. 66 677 50 247 9 25. 9 180. a. 06 677 50 247 9 2a 4 0. a 66 677 e 50 247 9 41. 0

Bleached Redwood Kraft Bleached Softwood Pandla Kraft 700 a. 19 597 120 s 10 2s. 9 205- a. 10 597 120 308 10 29. 6 700- a. 36 51a 50 247 0 25. 5 310..-" '3. 36 513 50 247 9 25. 3 158. 3.36 513 50 347 9 27.2 0 3. 36 513 60 247 9 34.8-

Corrugated Bo'xboard- Broke 700. 3.85 603 160 50s 10 a0. 5 285- 3. 85 503 100 g 308 10 31.1 0 3. 85 503 160 30s 10 42. 5 Unbleached Softwood Kraft at 12.5 pH

In FIG. 14, the data obtained above for the bleached hardwood kraft that was processed through theappara- V tus of this invention at .1 87 times the force of gravity is shown in curve 138 while that which was processed at curvel44 while that which was processed at 187 times respect to various kinds of pulp which were processed through the aforementioned apparatus;

Feed Scroll- Product. consist- Feed Centrifscreen 0.D Feed eucy, Iree- Feed ugal rgmm. solids rate, percent ness temp. force ifierpercent g.p.m. (by wgt.) 0.3.1 1 factor ential (by wgt.)

Bleached Hardwood Kraft the force of gravity is shown in curve'l46. The data obtained above with respect to the bleached softwood kraft which was processed at.247 times the force of gravity is shown in curve 148. The data obtained above with respect to bleached redwood kraft which was processed at.247 times the force ofgravity is shown in curve 150. The data obtained above with respect to the bleached softwood pandia kraft which was processed at 308 times the force of gravity is shown in curve 152 while that which was processed at 247 times the force of gravity is shown in curve 154. The data obtained above with respect to corrugated boxboa'rd broke which was processed at 308 times the force of gravity is shown in curve 156. Finally, the data obtained above with respect to unbleached softwood kraft at l2.5 Ph which was processed at 308 times the force of gravity is shown in curve 158. From the foregoing, it will be readily apparent that a high consistency product was obtained from a low consistency pulpeven at high feed rates of approximately 700 gallons per minute. It will V be noted that although FIG. 14 shows feed rates only from gallons per minute to 700 gallons per minute the data listed above was also obtained at zero feed rate, i.e., the consistency of the fibrous material contained within the apparatus following a complete shutdown thereof. In this connection there nearly always remains within the apparatus a small amount of fibrous material following shut down thereof.

F i0. shows the optimum centrifuge gravity forces and related power requirements for maximum product dryness at given feed rates. The information reflected in the graph shown in F l6. 15 was obtained by processing bleached. hardwood kraft pulp through an apparatus constructed in accordance with the subject invention. The feed consistency of the bleached hardwood kraft varied between approximately 3.1 and 4.1 per cent. The Canadian StandardFreeness (C.S.F.) varied between 573 and 595. The Canadian Standard Freeness is a value, expressed in millimeters, indicating the ease with which a pulp will drain. The larger the C .S.F. number for a pulp means the easier it will drain. The temperature of the bleached hardwood kraft was 50F.

' In this connection, it is to be noted that the product consistency increased with an increase in the centrifugal force at which same was being centrifuged up to a certain point and thereafter even though the centrifugal force at which same was being centrifuged was increased, the product consistency decreased.

One of the difficulties encountered in attempting to remove liquid from a fibrous material through the use of a centrifuge was that of experiencing relatively large losses of fiber in the effluent. in an attempt to reduce the amount 'of fiber being lost in the effluent, the size of the screen openings was reduced. Unfortunately, when the size of the screen openings was reduced sufficiently to reduce to an acceptable minimum the amount of fiber being lost in the effluent, it was found that the production capacity of the centrifuge was reduced so drastically that, from an economic viewpoint,.

same could not be considered for use in pulp mills for removing liquid from a fibrous material. It has been found, however, that the amount of fiber being lost to the effluent may be substantially reduced through the use of a second screen means disposed in encompassing relationship about the first screen means and generally spaced apart therefrom. For example, in H6. 16 the line 160 shows the amount of fiber material passing with the liquid through the screen opening. The points indicated on line 160 represents the data obtained for various kinds of pulps. Thus, the line 160 represents the amount of fiber contained in the primary'effluent, i.e.,

the effluent passing through the first or main screen means. The points shown between lines 162 and 164 represent the amount of fiber contained in the secondary effluent, i.e., the effluent passing through the second screen means. More specifically, the fiber content of a primary effluent having a consistency of approximately.0.2l per cent (approximately 17 lbs. of fibrous stock per 1,000 gallons of primary effluent) will, upon passing through the second screen means, be reduced to approximately 0.05 per cent consistency (approximately 4 lbs. of fibrous stock per 1,000 gallons of sec-v ondary effluent). Thus, the data shown between lines 162 and 164 represent the amount of fibrous stock contained in the secondary effluent. The second screen means used to obtain the data shown in FIG. 14 had conically shaped openings formed therein, the diameter of said openings increasing in a direction proceeding from the inner surface of said screen means to the outer surface thereof. The minimum diameter of said openings and the thickness of said screen means equalled approximately 0.125 millimeters. Although the use of awoven wire screen or a cloth screen as a part of the main screen means is normally to be avoided, it has been determined that same may be advantageously used in the second screen means.

Heretofore, the results obtained by processing a short fibered pulp (characterized as being produced by a two or three pass stone or refiner groundwood process) through'the use of a continuous centrifuge were even more discouraging than those obtained with respect to the relatively long fibered pulps. For example. the consistency of short fibered pulp processed through a centrifuge remained essentially unchanged. i.e. where a feed consistency of approximately 3.5 per cent was in volved the product consistency might increase only by an amount of l or 2 percent. It has been found, however, that liquid may be easily removed from a short fibered material by adding to said material a predetermined quantity of fibers which are relatively long in length, then impinging said mixture upon a rotating screen until substantially all of the fibrous material contained therein forms a layer on said surface while passing a portion of the liquid through said screen, followed by effecting a further removal of liquids from the mixture by cleaning the screen surface by moving the surface of a transporting member adjacent said screen surface while simultaneously producing and maintaining a mass of densely compacted, relatively impervious fibrous material adjacent one side of said member and between said surfaces and'passing additional portions of the liquid through the screen. The quantity of relatively'long fibers added to the short fibered material must be sufficient to effect the producing and maintaining of the mass of densely compacted, relatively impervious fibrous material while at the same time reducing the amount of fiber lost in the effluent. It has been found that where an insufficient amount of long fibers is involved, it is impossible to form the mass of relatively impervious fibrous material, unless the flight edges were located prohibitively close to the inner surface of the screen means, due to very low cohesiveness of the short fibered material. However, by adding even a small quantity of long fibered material, said mass is easily formed and maintained. Additionally, where short fibered material is involved, the amount of fiber lost in the effluent iner eases with a decrease in the length of the fibers. However, this condition is overcome by adding a small quantity of long fiber material since the long fibers assist in forming the layer of material while substantially reducing the amount of fiber passing through the screen openings. Good results are obtained when the amount of relatively long fibers equals atleast approximately one-half of 1 percent, by weight, of all the fibrous material contained in the mixture. In FIG. 17 is shown the results of processing short fibered material in accordance with the method and apparatus of this invention by the addition of predeterapproximately F. The ease with which the fibrous.

material could be drained was very poor, the Canadian Standard Freeness being only 82. It will be understood that different types of fibers have different lengths.

However, as used herein a short fibered material refers 4 clockwise direction as viewed from the first end at a w to a fiber produced by a two or three pass stone or refiner groundwood. Such fibers are short enough to pass rial is dependent, in part, upon the amount of time that the fibrous material is retained upon the inner surface of the rotating screen means. The amount of time required for fibrous material to pass through an apparatus constructed in accordance with the subject invention is dependent upon a number of factors. For example, the retention time is directly proportional to the length of the screen means. The retention time is also dependent upon the angle formed between the helical flights and a plane disposed perpendicular to the longitudinal'axis of the scroll member and the difference in angular speeds between the scroll member and the screen means. In this connection, it will be noted that normally the angular velocity differential will by necessity be mechanically increased with a decrease in the flight angle as aforesaid if the retention time is to remain unchanged. Another factor affecting the retention time is the coefficient of friction of the screen and the coefficient of friction of the helical flight surfaces. In this connection, the retention time is increased with an increase in the coefficient of friction of the inner surface of the screen and decreased with a decrease in the coefficient of friction of the helical flight surfaces. Since the efficiency of the method and apparatus of this invention is dependent upon the retention period, in

sults are obtained where the retention time is between approximately 1 /2 seconds and 3 and 6/ 10 seconds. For many applications, it has been found that a retention period of approximately 3 seconds provides optimum results. In connection with references made herein with respect to the'period of time that the fibrous material must becentrifuged or retained upon the main screen means, it is to 'be understood that such periods are based upon the amount of time calculated for the fibrous material to move along the inner surface of the main screen means from one end, thereof to the other in which consideration is given only to the angular velocity differential existing'between the screen means and the scroll member, the angle of inclination that the helical flights are formed upon the scroll member, and

the length of said screen means. More specifically, the retention time periods referred to herein are based upon calculations which are made without-regard to the effect of the force of friction.

Referring now to FIG. 1, a fibrous material mixture is fed to the apparatus through the feed inlet 26. The fibrous material mixture falls upon the upper surface of the scroll member 24 and is immediately centrifuged radially outwardly to the inner surface of the screen means 22. The fibrous material contained within the mixture is retained upon the inner surface of the screen means 22 while some of the liquid passes through the openings 52 formed therein. Since the scroll member and its associated helical flights are rotated in a counter slightly slower angular velocity than the counter clockwise rotating screen means, it will be understood that relative movement between the edges of the helical flights and the inner surfaceof the screen means is effected. Movement of the helical flight relative to the inner surface of the screen means functions to remove the layer of fibrous material retained on the inner surface of the screen means. The fibrous material is centrifuged at a forceto permit a layer of fibrous material to be fonned upon the inner surface of the screen means which will be sufficiently porous to pemtit the passing of a portion of the liquid through said layer of material and through the screen. This condition is met for most pulps where the centrifugal force varies within a range between approximately 70 to 650 times the force of gravity. As the helical flights move relative to the inner surface of the screen means, a mass of densely compacted, relatively impervious fibrous material is formed and maintained adjacent one side of the edge of the helical flight and between opposed surfaces of the helical flights and the inner surface of the screen means. This mass of relatively impervious material is used to remove continuously the layer of fibrous materialdisposed on the inner surface of the screen means. As the fibrous material is removed from the inner surface of the screen means, it moves along the adjacent surface of the helical flight or, following formation of said mass of relatively impervious material, along the mass of relatively impervious material following which same undergoes a rolling and wringing action due to the continued application of the centrifugal force all of which results in effecting a further removal of liquid from the mixture. The liquid removed from the material being fed into the apparatus 20 passes through the screen means 22 and into the liquid receiving means 28 from which same is continuously being removed. The fibrous material deposited upon the inner surface of the screen means passes along the inner surface of the screen means generally longitudinally and circumferentially thereof into the fibrous material receiving means area 30.v In the event it is desirable or necessary to wash or treat with a liquid the fibrous material following removal of a portion of the liquid therefrom, said washing or treatment is effected by feeding a liquid through the conduit 170 which is disposed incommunication with the interior of the scroll member 24. The liquid passes through the interior of the scroll means 24, through an I opening 172 formed in the radially extending plate 174 into the lower portion 176 of the scroll member 24. The liquid then flows radially outwardly through a plurality of openings 178 formed in the outer surface of the scroll member 24. The liquid passing through the opening 178 comes into contact with the fibrous material disposed on the inner surface of the lower portion of the screen means 22 and thence into the liquid receiving means 28. The liquid collected within the liquid receiving means 28 is continuously removed through tbs Openin Q9.-

It will be understood that the fibrous material may be washed or treated with a liquid or treated with a gas by v that all of the gas is moved along with the fibrous material.

It will be understood that where a second screen means 23 is used in conjunction with the screen means 22 as shown in H6. 1, suitable means 182 will be provided for collecting the liquid passing through said second screen means. The fibrous material deposited upon the inner surface of the second screen means 23 will pass into and be collected in the means 28. Although the fibrous material so collected on the inner surface of the second screen means 23 need not be processed again through the apparatus 20, it has been found desirable to do so in many circumstances for the purpose of increasing the consistency of the material collected on the inner surface of the second screen means.

Referring again to FIG. 10, it will be noted that the assembled scroll member and screen means is concentrically mounted within a second screen means 170 which has a conically shaped inner surface. Additionally, it will be noted that the diameter of said second screen means 170 decreases in the direction of flow of the fibrous material through the apparatus. The purpose of forming the second screen means 170 in this manner is to increase the retention period that the fibrous material passing through the openings formed in the first screen means 106 is retained upon the inner surface of the second screen means 170. By increasing the amount of time that the fibrous material is retained upon the inner surface of the second screen means 170, it is possible to remove an increased amount of liquid thereby increasing the consistency of the fibrous material collected from the inner surface of the second screen means 170.

In view of all of the foregoing, it will now be readily appreciated that a method and apparatus for continuously removing liquid from a mixture by centrifuging same has been described. Further, the method and apparatus of this invention permits the continuous removal of large quantities of liquid from a fibrous mate rial mixture in a most efficient manner with equipment which is relatively inexpensive to manufacture. Additionally, a single centrifuge constructed in accordance with the subject invention may be used to handle large volumes of fibrous material mixture. The life of a centrifuge constructed in accordance with the subject invention has been considerably increased due to the reduced speeds at which same may be operated, the use of a relatively thick screen means which permits the efficient removal of liquid from a fibrous material mixture and, if desired, the coating of the flight edges with elastomeric material as specified herein. The method and apparatus of this invention is additionally enhanced through the use of a second screen means for substantially reducing the overall amount of fiber which would otherwise be lost in the effluent. Construction of the screen means is simplified through the use of an outer, encompassing perforated basket having a longitudinally extending recess formed therein and in which is disposed overlapping ends of an inner screen member. The method and apparatus of this invention is further enhanced by recognizing and controlling the amount of time that the fibrous material is centrifuged. Moreover, the method and apparatus of this invention may be beneficially used to remove liquid from a very low consistency fibrous material mixture and, by adding a small quantity of long fibered material, the method and apparatus may be used to remove liquid from a short fibered fibrous material.

It is to be understood that this invention is not limited to the exact methods and embodiments shown and described, which are merely by way of illustration and not limitation, as various other methods. forms and modifications will be apparent to those skilled in the art. For example, it will be understood that an apparatus of this invention may be used in combination with gravity or vacuum filters as well as with presses. Therefore. it is intended that the appended claims cover all such changes and modifications.

We claim: I

l. A method of continuously removing liquid from an organic cellulose fiber-liquid mixture such as paper pulp comprising the steps of:

a. continuously feeding said mixture into one end of an annular first screen having openings formed therethrough, the minor dimension of each opening being between 0.3 and 3.0 mm;

b. rotating said first screen at a rate sufficient to develop a centrifugal force varying between 70 and 650 times the force of gravity;

c. impinging said mixture against the inner surface of said first screen thereby removing liquid from said mixture and passing said liquid so removed through the openings of said first screen while collecting fibers on the inner surface of said first screen;

d. continuously scraping the collected fibers from the inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing same on the inner surface of said first, screen while simultaneously urging the fibers toward the other end of said first screen and removing additional liquid from said mixture and passing said additional liquid through the openings of said first screen, and

e. continuously removing the fibers from the other end of said first screen. v V 7 n 2. A method as described in claim 1 including the step of impinging said mixture against the inner surface of said first screen and scraping fibers from said inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing the rolled and wrung fibers on the inner surface of said first screen for a time period between 1% to 12 seconds.

3. A method as described in claim 1 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.

4. A method as described in claim 3 in which the step of entrapping and collecting fibers includes impinging said fibers and liquid passing through the openings of said first screen upon the inner surface of said second screen at an angle obliquely disposed with respect to the surface of said second screen. I

5. A method as described in claim 1 in which the step of rotating said first screen includes rotating said first screen at a rate sufficient to develop a centrifugal force between and 550 times the force of gravity.

6. A method as described in claim 1 in which the step of rotating said first screen includes rotating said first screen at a rate sufficient to develop a centrifugal force between I40 and 450 times the force of gravity.

7. A method of continuously removing liquid from an organic cellulose fiber-liquid mixture, such as paper pulp, in which a substantial portion of said fibers are minus 16 mesh in length, said method comprising the a mixture in which at least approximately one-half of lper'cent of the fibers have a length of at least 1% millimeters;

b. continuously feeding said mixture into one end of an annular first screen having openings formed therethrough, the minor dimension of each opening being between 0.3 and 3.0 mm;

c. rotating said first screen at a rate sufficient to develop a centrifugal force varying between 70 and (150 times the force of gravity;

, d. impinging said mixture against the inner surface of said first screen thereby removing liquid from said mixture and passing said liquid.

so removed through the openings of said first screen while collecting fibers on the inner surface of said first screen;

e. continuously scraping the collected fibers from the inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing said rolled and wrung fibers on the inner surface of said first screen while simultaneously urging the fibers toward the other end of said first screen; and

f. continuously removing the fibers from the other end of said first screen;

8. A method as described in claim 7 including the step of impinging said mixture against the inner surface of said first screen and scraping fibers from said inner surface of said first screen, rolling and wringing the scraped fibers, and redepositingthe rolled and wrung fibers on the inner surface of said first screen for a time period between 1 /2 to 12 seconds.

9. A method as described in claim 7 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.

10. A method of continuously removing liquid from a low consistency organic cellulose fiber-liquid mixture such as paper pulp comprising the steps of:

a. centrifuging said mixture having a consistency, by weight, between one-half and twelve per cent at a centrifugal force between 70 and 650 times the force of gravity;

b. impinging the centrifuged mixture against the inner surface of an annular, rotating screen having openings formed therethrough, the minor dimension of said openings being between 0.3 and 3.0

mm, while simultaneously removing from said mix-' c. continuing the centrifuging and impinging steps and simultaneously removing an additional part of said liquid and passing the additional part of liquid so removed through the openings of said screen by continuously scraping the fibers from the inner surface of said screen. rolling and wringing said scraped fibers, and redepositing same on the inner surface of said screen while simultaneously urging the fibers spirally along the inner surface of said screen; and d. continuously-removing the fibers from the inner surface of said screen. ll. A method as described in claim 10 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.

12. A method as described in claim 10 in which the impinging step includes impinging the centrifugal mixture against theinner surface of an annular, rotating screen having an effective open area of at least 10 percent.

13. A method of continuously removing liquid from an organic cellulose fiber-liquid mixture such as paper pulp comprising the steps of:

a. continuously feeding said mixture into one end of an annular rotatably mounted screen having openings formed therethrough, the minor dimension of each opening being between 10 and 50 times the diameter of the fibers of said mixture;

b. rotating said screen;

c. impinging said mixture against the inner surface of said screen thereby removing liquid from said mixture and passing said liquid so removed through the openings of said screen while collecting fibers on the inner surface of said screen;

d. continuously scraping the collected fibers from the inner surface of said screen, rolling and wringing the scraped fibers, and redepositing same on the inner surface of said screen while simultaneously urging the fibers toward the other end of said screen and removing additional liquid from said mixture and passing said liquid through the openings of said screen; and

. continuously removing the fibers from the other end of said screen. 

2. A method as described in claim 1 including the step of impinging said mixture against the inner surface of said first screen and scraping fibers from said inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing the rolled and wrung fibers on the inner surface of said first screen for a time period between 1 1/2 to 12 seconds.
 3. A method as described in claim 1 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.
 4. A method as described in claim 3 in which the step of entrapping and collecting fibers includes impinging said fibers and liquid passing through the openings of said first screen upon the inner surface of said second screen at an angle obliquely disposed with respect to the surface of said second screen.
 5. A method as described in claim 1 in which the step of rotating said first screen includes rotating said first screen at a rate sufficient to develop a centrifugal force between 100 and 550 times the force of gravity.
 6. A method as described in claim 1 in which the step of rotating said first screen includes rotating said first screen at a rate sufficient to develop a centrifugal force between 140 and 450 times the force Of gravity.
 7. A method of continuously removing liquid from an organic cellulose fiber-liquid mixture, such as paper pulp, in which a substantial portion of said fibers are minus 16 mesh in length, said method comprising the steps of: a. adding to said mixture a quantity of fibers to form a mixture in which at least approximately one-half of 1 percent of the fibers have a length of at least 1 1/4 millimeters; b. continuously feeding said mixture into one end of an annular first screen having openings formed therethrough, the minor dimension of each opening being between 0.3 and 3.0 mm; c. rotating said first screen at a rate sufficient to develop a centrifugal force varying between 70 and 650 times the force of gravity; d. impinging said mixture against the inner surface of said first screen thereby removing liquid from said mixture and passing said liquid so removed through the openings of said first screen while collecting fibers on the inner surface of said first screen; e. continuously scraping the collected fibers from the inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing said rolled and wrung fibers on the inner surface of said first screen while simultaneously urging the fibers toward the other end of said first screen; and f. continuously removing the fibers from the other end of said first screen;
 8. A method as described in claim 7 including the step of impinging said mixture against the inner surface of said first screen and scraping fibers from said inner surface of said first screen, rolling and wringing the scraped fibers, and redepositing the rolled and wrung fibers on the inner surface of said first screen for a time period between 1 1/2 to 12 seconds.
 9. A method as described in claim 7 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.
 10. A method of continuously removing liquid from a low consistency organic cellulose fiber-liquid mixture such as paper pulp comprising the steps of: a. centrifuging said mixture having a consistency, by weight, between one-half and twelve per cent at a centrifugal force between 70 and 650 times the force of gravity; b. impinging the centrifuged mixture against the inner surface of an annular, rotating screen having openings formed therethrough, the minor dimension of said openings being between 0.3 and 3.0 mm, while simultaneously removing from said mixture a portion of the liquid and passing the liquid so removed through the openings of said screen; c. continuing the centrifuging and impinging steps and simultaneously removing an additional part of said liquid and passing the additional part of liquid so removed through the openings of said screen by continuously scraping the fibers from the inner surface of said screen, rolling and wringing said scraped fibers, and redepositing same on the inner surface of said screen while simultaneously urging the fibers spirally along the inner surface of said screen; and d. continuously removing the fibers from the inner surface of said screen.
 11. A method as described in claim 10 further comprising the step of entrapping and collecting those fibers passing through the openings of said first screen upon the inner surface of a second screen mounted concentrically about the outer surface of said first screen.
 12. A method as described in claim 10 in which the impinging step includes impinging the centrifugal mixture against the inner surface of an annular, rotating screen having an effective open area of at least 10 percent.
 13. A method of continuously removing liquid from an organic cellulose fiber-liquid mixture such as paper pulp comprising the steps of: a. continuously feeding said mixture into one end oF an annular rotatably mounted screen having openings formed therethrough, the minor dimension of each opening being between 10 and 50 times the diameter of the fibers of said mixture; b. rotating said screen; c. impinging said mixture against the inner surface of said screen thereby removing liquid from said mixture and passing said liquid so removed through the openings of said screen while collecting fibers on the inner surface of said screen; d. continuously scraping the collected fibers from the inner surface of said screen, rolling and wringing the scraped fibers, and redepositing same on the inner surface of said screen while simultaneously urging the fibers toward the other end of said screen and removing additional liquid from said mixture and passing said liquid through the openings of said screen; and e. continuously removing the fibers from the other end of said screen. 